This application claims the benefit of the Korean Application Nos. P2001-028959 filed on May 25, 2001, and P2001-030861 filed on Jun. 1, 2001, which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a method of forming polycrystalline silicon for a liquid crystal display device.
2. Discussion of the Related Art
A thin film transistor liquid crystal display (TFT-LCD) has been developed to have a high device packing density and a large sized screen, and to form the display part and the driving circuit part on the same substrate. In order to meet the requirements, a mobility of the thin film transistor should be improved. However, it is difficult to enhance the mobility by using an amorphous Si:H thin film transistor (Si:H TFT).
Recently, as a solution for the problem, a polycrystalline silicon TFT (Poly-Si TFT) has been paid much attention. Since the polycrystalline silicon TFT has a great mobility, it has an advantage in that the peripheral circuits can be integrated on the insulating substrate, and a TFT production cost is substantially reduced.
Moreover, as the polycrystalline silicon TFT has a mobility greater than an amorphous silicon TFT, the polycrystalline silicon TFT is favorable for a switching device for a high resolution panel. Also, it is suitable for a projection panel receiving much external light since the polycrystalline silicon TFT has less photo-current unlike the amorphous silicon TFT.
There are many reported methods for forming the polycrystalline silicon. The methods can be sorted as a method for depositing polycrystalline silicon directly and a method for depositing amorphous silicon and crystallizing the amorphous silicon to convert to polycrystalline silicon.
In the former method, there are a low pressure chemical vapor deposition (LPCVD) method, a plasma enhanced chemical vapor deposition (PECVD) method, and the like. The LPCVD method uses expensive silica or quartz as a substrate because the LPCVD method has a high deposition temperature of 550xc2x0 C. Therefore, the LPCVD method is not suitable for mass production because of the high production cost.
Although the PECVD method is possible to deposit polycrystalline silicon at a temperature below 400xc2x0 C. by using a mixture gas of SiF4/SiH4/H2, it is very difficult to suppress crystalline grains. Moreover, it is known that the PECVD method causes a serious problem in a surface characteristic of a polycrystalline silicon thin film due to non-uniformity in the growth direction in the deposition.
In the latter method (i.e., in the method for depositing amorphous silicon and crystallizing the deposited amorphous silicon), there are a solid phase crystallization (SPC) method, and an excimer laser annealing (ELA) method.
The ELA method, in which a high energy excimer laser beam is directed to an amorphous silicon thin film in a form of pulses to crystallize the thin film in a moment, can form a polycrystalline silicon thin film having large crystalline grains with excellent crystalline characteristics.
However, as the ELA requires the excimer laser, which is an expensive additional equipment, the ELA has limitations in mass production and fabricating a TFT for driving a large sized LCD.
The SPC method, in which an amorphous silicon thin film is crystallized by heating in a furnace, has a slow crystallizing reaction rate because the reaction progresses in a solid state, even if polycrystalline silicon having excellent crystalline characteristics is formed. The SPC method requires a long time of crystallization of a few tens of hours at an elevated temperature higher than 600xc2x0 C.
Besides the foregoing methods, recently there have been many researches for lowering a crystallizing temperature for using polycrystalline silicon in fabricating a large sized LCD. One of the methods includes a metal induced crystallization (MIC) method. Also, there has been a research for a field effect metal induced crystallization (FEMIC) method.
According to the foregoing methods, it is known that a crystallization temperature of amorphous silicon can be lowered to a temperature below 500xc2x0 C. if a particular kind of metal is brought into contact with the amorphous silicon.
Causes of the metal induced crystallization vary with different kinds of metals. That is, a crystallization may vary with different metals that are in contact with a-Si:H. For an example, metal induced crystallizations of aluminum (Al), gold (Au), or silver (Ag) are limited by the diffusion of silicon at an interface of the metal with amorphous silicon. That is, the diffusion of the silicon at the interface of the metal with the amorphous silicon forms a silicide phase of a metastable state by a silicon diffusion. Silicide serves to lower a crystallization energy, thereby accelerating the crystallization of silicon.
On the other hand, a metal induced crystallization of nickel (Ni), or titanium (Ti) is controlled by the diffusion of metal caused by annealing at an interface of the metal with the amorphous silicon. That is, the diffusion of the metal at the interface of the metal with the amorphous silicon into a silicon layer forms a silicide phase. Silicide serves to accelerate the crystallization of silicon and drop the crystallization temperature.
A related art method of crystallizing an amorphous silicon film will be explained, with reference to the attached drawings. FIGS. 1A to 1C illustrate schematic cross-sections showing a related art method of crystallizing an amorphous silicon thin film.
Referring to FIG. 1A, an insulating film 102 is formed on an insulating substrate 101 as a buffer layer. An amorphous silicon layer 103 is deposited on the buffer layer 102 as an active region. Then, a catalyst metal thin film 104, serving as a crystallization catalyst, is formed on the amorphous silicon layer 103.
Referring to FIG. 1B, a pair of electrodes 105 are formed on the catalyst metal thin film 104 for applying electric fields.
Then, referring to FIG. 1C, when the electrodes 105 are heat-treated at approximately 500xc2x0 C., with electric fields applied thereto, clusters of the catalyst metal are diffused toward the amorphous silicon layer 103, resulting in forming silicide NiSi2. The silicide NiSi2 accelerates crystallization of the amorphous silicon 103, thereby crystallizing the amorphous silicon layer 103 into a polycrystalline silicon layer 106 since a crystallization temperature is dropped. That is, amorphous silicon is crystallized by the FEMIC effect.
However, the related art method of forming polycrystalline silicon has the following problems.
Referring to FIG. 1C, there are unreacted metal clusters 104a of the catalyst metal clusters remained on a surface of the polycrystalline silicon formed by crystallization of the amorphous silicon. The remained catalyst metal clusters cause a leakage current in a following process, such as deposition of an insulating film, thereby deteriorating a device performance. That is, when a gate insulating film of the thin film transistor is formed, the catalyst metal remained on the surface of the polycrystalline silicon formed by FEMIC causes a fixed charge at the interface of the polycrystalline silicon and the gate insulating film. The fixed charge deteriorates device performance, such as shifting a threshold voltage of the active region, and increasing an off current Ioff. Moreover, a heat loss is caused at a surface of the amorphous silicon and slows down a crystallization rate. Thus, a point defect is generated in the grain of the polycrystalline silicon after the crystallization.
Accordingly, the present invention is directed to a method of forming polycrystalline silicon for a liquid crystal display device that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
Another object of the present invention is to provide a method of forming polycrystalline silicon for a liquid crystal display device, which controls an amount of catalytic metal clusters remained in the polycrystalline silicon.
A further object of the present invention is to provide a method of forming polycrystalline silicon for a liquid crystal display device, which minimizes an amount of unreacted catalytic metal remained in the polycrystalline silicon.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for forming polycrystalline silicon from amorphous silicon includes forming an amorphous silicon layer on a substrate, forming a plurality of catalytic metal clusters on the amorphous silicon layer, forming a catalytic metal gettering layer adjacent to the amorphous silicon layer, and heat-treating the substrate including the amorphous silicon layer to transform the amorphous silicon layer into a polycrystalline silicon layer, wherein unreacted catalytic metal clusters migrate to the catalytic metal gettering layer in a direction perpendicular to the substrate.
According to another aspect of the present invention, a method of forming polycrystalline silicon from amorphous silicon includes forming an amorphous silicon layer on a substrate, forming a plurality of catalytic metal clusters on the amorphous silicon layer, simultaneously applying electric fields to the catalytic metal clusters and first heat-treating the substrate to transform the amorphous silicon layer into a polycrystalline silicon layer, forming a catalytic metal gettering layer on the polycrystalline silicon layer, second heat-treating the substrate including the polycrystalline silicon layer to getter unreacted catalytic metal clusters migrated at the catalytic metal gettering layer, and removing the catalytic metal gettering layer including the unreacted catalytic metal clusters.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.